Conventional dual pressure processes are employed to separate air at cryogenic temperatures into oxygen and nitrogen. Air is first compressed to approximately 5-6 atm absolute and then subjected to rectification in a high and low pressure distillation column which are thermally linked to one another. The high pressure column operates under superatomospheric pressure corresponding to the pressure of the air feed. The air feed undergoes preliminary separation in the high pressure column into a liquid fraction of crude oxygen and a liquid fraction of substantially pure nitrogen. The two resulting liquids typically form the feed fraction and the rectification reflux for the low pressure distillation operation.
The relative volatilities of nitrogen and oxygen force oxygen to accumulate at the bottom stripping section of the low pressure distillation and nitrogen to accumulate at the top of the low pressure distillation.
Specifically, liquid and vapor are passed in counter-current contact through one or more columns and the difference in vapor pressure between the oxygen and nitrogen cause nitrogen to concentrate in the vapor form and oxygen to concentrate in the liquid form. The lower the pressure in the separation column, the easier it is to separate air into oxygen and nitrogen due to higher relative volatilities. Accordingly, the final separation into product oxygen and nitrogen is generally carried out at a relatively low pressure, usually just a few pounds per square inch (psi) above atmospheric pressure.
The consistent production of oxygen and nitrogen require that the composition variables of the cryogenic air separation process remain constant throughout the production cycle. It has been observed, however, that disturbance causing a deviation in any one of the composition variables may change the process sufficiently so that inferior quality oxygen is produced and/or a reduction in the rate of production is encountered. This results in the inefficient operation of the cryogenic air separation process and in the production of poor quality oxygen product or reduced product oxygen flow.
To insure that the quality of the product produced and the efficiency of the process is maintained would require constant monitoring of the output rate and quality of product produced. An alternate source of product such as liquid which is vaporized when either the output rate or product quality deviate from a specified value is generally required. This approach is costly and time consuming and thereby an inefficient solution to the problem.
It is an object of the present invention to provide a cryogenic air separation process that can produce oxygen having a desired purity composition on a continuous basis minimizing or eliminating the need for an alternative source of product.
Another object of the present invention is to provide a cryogenic air separation process that employs an interstage condenser/reboiler that is automatically monitored and the data observed are compared with preselected data so that any deviation between the measured and preselected data will produce a control signal that can be used to adjust at least one of the input and/or output feeds of the system so that the quality and/or feed rate of the product is returned to its desired levels.
Another object of the present invention is to provide a cost effective and easy to operate process for producing oxygen and nitrogen from a cryogenic air separation system on a continuous basis.
The foregoing and additional objects will become fully apparent from the following description and drawings.